Hydraulic fracturing (also referred to as fracking and/or hydrofracking) injects pressurized water, usually containing sand and chemicals (e.g., surfactants), into wellbores with the intent of stimulating hydrocarbon production. This mixture is referred to as fracking fluid. Fracking fluid is injected primarily into shale formations to break apart the shale. The sand enters the resulting cracks, to keep open pathways for natural gas or oil to flow to the surface. Although the fracking process is used to generate an abundant flow of petroleum products, including methane, the process forces significant amounts of contaminated water to the surface as wastewater.
Wastewater generated in these operations includes both flowback water and produced water. Flowback water is defined as a mixture of fracking fluid plus formation water (water present in the shale rock formation) and is the first water generated by the well. The chemical composition of flowback water resembles that of the fracking fluid. Produced water follows flowback water and resembles more closely the composition of water present in the shale formation. Produced water is generated throughout the life of the well, in some cases discharging up to 2,500 barrels per day.
Both flowback and produced water are contaminated with a range of inorganic and organic chemicals and must be treated before releasing them to the environment. The composition of flowback and produced water varies from well to well, and also varies over the life of the well. Contaminants in both flowback and produced water may include suspended solids, dissolved material, and/or organic matter. Suspended solids may include sand, dirt, and/or insoluble metal complexes. Dissolved material may include inorganic cations and anions such as cations of barium, calcium, iron, magnesium, potassium, sodium and strontium, and anions of carbonate, chloride and sulfate. The concentration of dissolved material associated with produced water can be up to and in excess of to 250,000 mg/l. Organic matter may include dissolved compounds and/or dispersed oils. Dissolved compounds comprise organic compounds present in the water and may include organic acids (e.g., formic and propionic acids), aromatic hydrocarbons (e.g., benzene, ethylbenzene, toluene, and/or xylenes), polyaromatic hydrocarbons, and/or phenols. For the purpose of this document, ammonia and amines are referred to as dissolved organic compounds. Dispersed oils include droplets of oil suspended in flowback and/or produced water. If allowed to stand for a length of time, these droplets will rise to the surface of the water, forming a sheen. The total organic content of flowback and produced water can range up to and in excess of 1,500 mg/l.
A number of processes have been proposed for the treatment of flowback and produced water generated from oil, gas and oil-gas production fields. Said processes have included membrane filtration, distillation, evaporation ponds, adsorption and filtration, and chemical oxidation. The diverse composition of flowback water presents a burden on purification processes. This is because the process must be capable of removing suspended solids, dissolved material and organic matter to trace levels that will allow for re-use or release of the treated water. As a result, achieving target levels of purification while meeting cost and size constraints proves troublesome.